Co-pending patent applications entitled "Prosthetic Spinal Disc Nucleus" and "Spinal Anulus Cutter" were filed on the same day as the present application and are assigned to the same assignee.
The present invention concerns a surgical method for implanting a prosthetic spinal disc nucleus into a human spinal disc space. More particularly, it relates to the implantation of pillow shaped prosthetic spinal disc nucleus bodies into a degenerated intervertebral disc space.
The vertebrate spine is the axis of the skeleton on which all of the body parts "hang". In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation.
The typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The centra of adjacent vertebrae are supported by intervertebral discs. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch which extends posteriorly and acts to protect the spinal cord's posterior side is known as the lamina. Projecting from the posterior region of the neural arch is the spinous process.
The intervertebral disc primarily serves as a mechanical cushion permitting controlled motion between vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulpous ("nucleus"), the anulus fibrosus ("anulus") and two vertebral end plates. The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus acts to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring which binds together adjacent vertebrae. The fibrous portion, which is much like a laminated automobile tire, measures about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the anulus consist of fifteen to twenty overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 40 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotates in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The healthy nucleus is largely a gel-like substance having a high water content, and like air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae while bending, lifting, etc.
The spinal disc may be displaced or damaged due to trauma or a disease process. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anulus confines. The mass of a herniated or "slipped" nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate back pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae are surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately this procedure places a greater stress on the discs adjacent to the fused segment as they compensate for lack of motion, perhaps leading to premature degeneration of those adjacent discs.
As an alternative to vertebral fusion, various prosthetic discs have been developed. The first prosthetics embody a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetics are all made to replace the entire intervertebral disc space and are large and rigid. Beyond the questionable applicability of the devices is the inherent difficulties encountered during implantation. Due to their size and inflexibility, these devices require an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve rootlets during posterior implantation cannot be avoided.
Anterior implantation, however, is highly suspect and introduces numerous risks. Various organs present physical obstacles as the surgeon attempts to access the damaged disc area. After an incision into the patient's abdomen, the surgeon is forced to engage the interfering organs and carefully move them aside. Ultimately the patient faces the brunt of the anterior approach risk should any organ be damaged.
An additional surgical concern, not previously addressed, is the potential damage imparted upon the anulus during implantation surgery. The normal anular plies act to keep the anulus tight about the nucleus. During surgery, a surgical knife or tool is used to completely sever some portion of the anulus and/or remove an entire section or a "plug" of the anulus tissue. When an entire section of the anulus is cut or removed to insert the prosthetic device, the layers making up the anulus "flay" and/or "pull back" and the constraining or tightening ability of that portion of the anulus is lost. Further, the chances of the anulus healing with restoration of full strength are greatly diminished, while the likelihood of nucleus reherniation is increased. An even greater concern arises where a significant portion of the anulus is removed entirely. A more desirable solution is to leave the anulus at least partially intact during and after implantation.
Recently, smaller and more flexible prosthetic nucleus bodies have been developed. With the reduction in prosthetic size, the ability to work around the spinal cord and nerve rootlets during posterior implantation has become possible.
While the posterior approach to intervertebral disc implantation does have potential difficulties, it is far more desirable than the anterior approach. Additionally, preserving the integrity of the anulus during implant enhances physical healing in the disc area. Therefore, a substantial need exists for a method of surgically implanting a prosthetic spinal disc nucleus body into the intervertebral disc space through a preferably posterior approach, with minimal damage to the anulus.